1. Field of the Invention
The invention relates to a commutator for small to medium-sized machines in which a number of circumferentially spaced segments having boundry surfaces defining an inner cover surface facing the longitudinal axis of the commutator directly abut an insulated rigid hub, and more particularly to such a commutator in which the segments form a body having at least one annular ring in which a biasing ring is arranged, and to a method for its manufacture.
2. Description of the Prior Art
The known commutators of this type are compression molded commutators in which the segment series is held together not only by the insulating molded material in which the segments are anchored, but also by reinforcing rings provided in annular grooves in the segment series to increase the dynamic load-bearing capacity of the commutator.
If the reinforcing rings are placed in the annular grooves with no bias, the increase in dynamic load-bearing capacity of the commutator is relatively small, because the reinforcing rings only assume the portion of the load by which the molded body is relieved by centrifugal force when the segment series begins to spread due to the centrifugal force.
But even if the reinforcing rings are placed into the annular grooves of a molded commutator with bias, no material increase in the dynamic load-bearing capacity of the commutator is achieved. This is due in part to the fact that the molded material shrinks more rapidly during cooling than the segment series, thus further reducing the ring bias tension as the spacing strips are removed which position the segments until the compression molding material is fully in place. Another significant reason is to be seen in the fact that the compression molding material, which is usually composed of resin and fillers, at least partially separates during the filling process, i.e., during the filling in of the segment series, whereby the already present nonhomogeneity of the components of the compression molding material employed is further increased, which leads to a distortion of the originally round outer surface during dynamic and thermal loading as a result of unequal tangential and radial bias.
Therefore, in those cases in which high dynamic and thermal loads occur, commutators of the compression arch construction are employed. In these commutators the segments are spaced by insulating laminae, which themselves undergo a plastic deformation during the molding of the segment series. The prebiased reinforcing rings product the requisite arch pressure and also effect the necessary forced closure with the hub or shaft. Until recently, it has been attempted to utilize the tension force of the reinforcing rings as much as possible for the production of the arch pressure, in order to obtain the highest possible arch pressure. The most recent high-performance commutators of the compression arch type construction, in contrast, use a significant proportion of the bias of the reinforcing rings to produce a radial prebiasing of the hub or shaft by means of a corresponding radial compression of the segments against the hub or shaft. However, just as before, even when cold, they exhibit an arch compression.